Illustration of the Compton-Belkovich Volcanic complex. Red is the volcanic complex; green area contains the radioactive debris from the eruption, which stretches 300km to the east. Image credit: Jack Wilson/Durham University

Mar 24, 2015
Ancient giant radioactive lunar volcano!

Sen—Every now and again, I get to have some fun writing a headline. Even better: This one’s true.

When you look at the Moon through a telescope, the history of asteroid and comet impacts is obvious; there are craters everywhere. The dark regions on the Moon’s face are called maria (the singular is “mare”, Latin for “sea”), and are places where magma bubbled up after particularly big impacts.

But there are a handful of true volcanoes as well. These have been dead for eons, a billion years at least if not more like 3-4 billion. They can be hard to identify, because countless impacts since then have eroded or covered up their identifying characteristics.

But in 1998 the orbiting Lunar Prospector spacecraft found something odd: a concentrated patch of radioactive thorium on the Moon’s far side. That’s common on the near side, concentrated in a huge region near Mare Imbrium and Oceanus Procellarum (a mare big enough to rate the name for “ocean”), and there is a big but relatively weak area near the south pole on the far side.

Gamma ray map of the Moon (near side on left, far side on right) showing the location of radioactive thorium. The volcano in question is marked by an arrow near the north pole on the far side. Image credit: NASA

The newly discovered spot, located between the craters Compton and Belkovich, is about 35 km across, and slightly more lightly colored than surrounding terrain. In 2011, using more recent observations by the Lunar Reconnaissance Orbiter, scientists determined this area was once a giant volcano. The spot features domes with steep sides, common in volcanoes that erupt thick, viscous lava, and some of the domes show collapse features as well, also common with big volcanoes.

The thorium found is interesting. It’s what geologists call an “incompatible element”; when magma cools to form rock, the thorium (along with several other elements) stay liquid longer. When the Moon formed, some minerals sank below the surface while others floated to form the crust. Thorium stayed in a layer between the two. Big impacts (like Imbrium/Procellarum) or volcanoes could bring it up to the surface. Some isotopes of thorium stay radioactive for billions of years, and can be detected even through a meter or so of rock. That’s how Lunar Prospector could see it.

That means the size of the volcanic eruptions can be determined. New analysis done of the Compton-Belkovich region show that thorium-laced lava was spewed out to distances of hundreds of kilometers! The eventual flow covered an area of 70,000 square kilometers, 3/4 the size of my home state of Colorado (or for anglophiles, a bit less than the area of Scotland). The eruption probably happened around 3.5 billion years ago, and the material has been softly glowing in radioactively-produced gamma rays ever since, waiting for us to discover it.

The thorium maps were fairly low resolution, but the scientists used a new technique (called the Markov Pixon Information method, which sounds like something you’d hear in a science fiction movie) to sharpen them up, making it easier to figure out how far the thorium reached. They think this technique can be used to work with other data, too, and hope to map Olympus Mons on Mars using the same method, though looking at maps of hydrogen and water ice instead of thorium.

I love this for so many reasons! The Moon is arguably the most and best-studied object in the sky, and yet it still holds surprises for us. And being able to write a headline like this one has its appeal, too.